Involvement of protumor macrophages in breast cancer progression and characterization of macrophage phenotypes

Tumor-associated macrophages (TAMs) are the most prominent immune cells in the breast cancer microenvironment, and the protumor functions of TAMs are thought to affect cancer progression and resistance to anticancer therapy. Numerous studies using human breast cancer samples, cell lines, and murine breast cancer models have revealed details of the mechanisms by which the protumor functions of TAMs are activated. Recent advances have highlighted the significant involvement of TAMs in the resistance of breast cancer cells to immunotherapy. Tumor-associated macrophages express a number of immunosuppressive genes, and single-cell sequence analyses of human and murine cancer samples have helped elucidate the mechanism of TAM-induced immunosuppression. As TAMs are considered suitable targets for anticancer therapies, we summarized the protumor functions of TAMs and the potential of anticancer therapies targeting TAMs, with a focus on breast cancer research.